N-hydroxy-N&#39;-nitrourea and related compounds as high energy density materials

ABSTRACT

Novel compounds are provided in the form of a salt having the structure of formula (I) or formula (II) 
     
       
         
         
             
             
         
       
     
     wherein: R 1  is selected from the group consisting of H, C 1 -C 24  alkyl, C 2 -C 24  alkenyl, C 2 -C 24  alkynyl, C 5 -C 20  aryl, C 6 -C 24  alkaryl, and C 6 -C 24  aralkyl; R 2  is H, C 1 -C 24  alkyl or nitro; X is O or NR 3  in which R 3  is H, C 1 -C 24  alkyl or nitro; Z m+  is a monovalent cation, a divalent cation, or a trivalent cation; M +  is an alkali metal cation; and m and n are 1, 2 or 3, with the proviso that the compound contains at least one nitro group. Exemplary such compounds are the ammonium salt of the N-hydroxyl-N′-nitrourea (NHNU) monoanion and the disodium salt of the NHNU dianion. Also provided, as novel compositions of matter, are N-hydroxyl-N′-nitrourea and N-hydroxyl-N-nitroguanidine in electronically neutral form. The compounds are useful in a variety of contexts, but are primarily useful as high energy oxidizing agents in explosive compositions, propellant formulations, gas-generating compositions, and the like. Compositions containing the compounds are also provided, including energetic compositions, as are methods for synthesizing the compounds.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119(e)(1) to Provisional U.S. Patent Application Ser. No. 60/310,707, filed Aug. 6, 2001, the disclosure of which is incorporated by reference herein.

REFERENCE TO GOVERNMENT SUPPORT

This invention was funded by the United States Air Force Office of Scientific Research under Contract No. F49620-00-C-0033. The United States Government has certain rights in this invention.

TECHNICAL FIELD

The present invention relates generally to energetic materials, and more particularly relates to novel chemical compounds useful, inter alia, as high energy oxidizing agents. Energetic compositions containing the compounds are also provided, as are methods for synthesizing the novel compounds.

BACKGROUND

In developing new energetic compounds, a number of factors come into play. For example, heat of formation, density, melting and decomposition temperatures, carbon content and, generally, nitrogen content, are properties that must be considered. Energetic compounds should display good thermal and shock properties and have high heats of formation. It is generally preferred that an energetic compound have a melting point above about 100° C., an exothermic heat of combustion, a positive heat of formation ΔH_(f), and a high decomposition temperature. A relatively large separation between melting point and decomposition temperature is preferred, so that an energetic composition may be melt cast from the selected compound. Finally, it is of course preferred that an energetic compound be relatively simple and straightforward to synthesize in high yield.

A number of energetic compounds are known to be useful as oxidizers, explosives, and the like. Energetic compounds have also been disclosed as useful to inflate automobile and aircraft occupant restraint bags. Previously known materials, however, are generally limited in one or more ways, e.g., they are overly impact-sensitive, difficult to synthesize on a large scale, not sufficiently energetic, excessively hygroscopic, or unstable at basic or slightly acidic pH. In addition, energetic compositions used to inflate occupant restraint bags in automobiles and aircraft typically contain potentially toxic heavy metal igniter materials, e.g., mercury compounds, Pb(N₃)₂, or the like.

The present invention provides a new class of compounds that overcomes the aforementioned limitations in the art. The energetic compounds to which the invention pertains are commonly referred to as “secondary” explosives, i.e., compounds whose energy is released after activation by initiator compounds, also termed “primary” explosives. The compounds now provided herein meet all of the above-mentioned preferred criteria, and outperform conventional energetic compounds in a number of ways. For example, higher O₂ density is provided than obtained with conventional secondary explosives such as ammonium nitrate. In addition, the novel compounds are highly energetic while not overly impact sensitive, and are straightforward to synthesize in high yield.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to address the above-mentioned need in the art by providing novel compounds that are useful as energetic materials, e.g., as high energy oxidizing agents.

It is still another object of the invention to provide energetic compositions containing one or more of the novel compounds.

It is yet another object of the invention to provide such energetic compositions in the form of propellant formulations, explosive compositions, and the like.

It is a further object of the invention to provide gas-generating compositions containing one or more of the novel compounds.

It is yet a further object of the invention to provide methods for synthesizing the novel compounds.

Additional objects, advantages, and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.

In one embodiment, the invention provides a compound in the form of a salt having the structure of formula (I)

wherein:

-   -   R¹ is selected from the group consisting of H, C₁-C₂₄ alkyl,         C₂-C₂₄ alkenyl, C₂-C₂₄ alkynyl, C₅-C₂₀ aryl, C₆-C₂₄ alkaryl, and         C₆-C₂₄ aralkyl;     -   R² is H, C₁-C₂₄ alkyl or nitro;     -   X is O or NR³ in which R³ is H, C₁-C₂₄ alkyl or nitro;     -   Z^(m+) is a monovalent cation, a divalent cation, or a trivalent         cation; and     -   m and n are 1, 2 or 3,     -   with the proviso that the compound contains at least one nitro         group.

In another embodiment, the invention is directed to a compound having the structure of formula (II)

wherein:

-   -   R¹ is selected from the group consisting of H, C₁-C₂₄ alkyl,         C₂-C₂₄ alkenyl, C₂-C₂₄ alkynyl, C₅-C₂₀ aryl, C₆-C₂₄ alkaryl, and         C₆-C₂₄ aralkyl;     -   R² is H, C₁-C₂₄ alkyl or nitro;     -   X is O or NR³ in which R³ is H, C₁-C₂₄ alkyl or nitro; and     -   M is an alkali metal,     -   with the proviso that the compound contains at least one nitro         group.

In another embodiment, the invention is directed to a compound having the structure of formula (III)

also referred to herein as N-hydroxyl-N′-nitrourea (NHNU), in electronically neutral form.

In still another embodiment, the invention is directed to a compound having the structure of formula (IV)

also referred to herein as N-hydroxyl-N-nitroguanidine, in electronically neutral form.

In another embodiment of the invention, energetic compositions are provided containing one or more of the novel compounds as energetic materials. These energetic compositions may take any number of forms and have a variety of uses, such as in rocket propellant formulations (including both solid and solution propellants), liquid monopropellants, bipropellant and tripropellant compositions, pyrotechnics, firearms, and the like. In addition, the compounds of the invention are useful in energetic, gas-generating compositions for inflating automotive or aircraft occupant restraint devices. As will be appreciated by those skilled in the art, the aforementioned uses are exemplary in nature and not intended to represent a comprehensive list of possibilities.

In a further embodiment, a method is provided for synthesizing N-hydroxyl-N′-nitrourea and salts and analogs thereof, by treating a basic salt of a lower alkyl nitrocarbamate and a monovalent cation with a hydroxide-releasing base in the presence of aqueous hydroxylamine until a salt of the cation and di-anionic N-hydroxyl-N′-nitrourea precipitates. The salt may be, for example, a sodium salt or a potassium salt. The electronically neutral form of N-hydroxyl-N′-nitrourea may then be obtained by processing the precipitate with an acid, preferably in the form of a sulfuric acid solution. The precipitate may or may not be isolated prior to processing with the sulfuric acid solution. The method may also include filtering the N-hydroxyl-N′-nitrourea obtained, and may further include crystallizing the filtered N-hydroxyl-N′-nitrourea. The ammonium salt of N-hydroxyl-N′-nitrourea, one preferred compound herein, may be prepared by redissolving the electronically neutral N-hydroxyl-N′-nitrourea obtained after filtering and crystallization in a solvent containing a stoichiometric amount of ammonia.

The compounds of the invention, as alluded to above, are advantageous in numerous ways. For example, the compounds:

-   -   are straightforward to synthesize and scale up, in high yield;     -   exhibit low impact sensitivity, on the order of 50% less than         that associated with RDX         (cyclo-1,3,5-tri-methylene-2,4,6-trinitramine; also referred to         as “cyclonite”) and HMX         ((1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane);     -   are powerful oxidizing agents, comparable to ammonium nitrate         (AN) in oxidizing power, while burning more readily than AN;     -   are substantially less hygroscopic than other high energy         density compounds, such as ammonium dinitramide (ADN), even at         90% relative humidity;     -   have a peak decomposition temperature (T_(d)) as determined by         Differential Scanning Calorimetry (DSC), of from about 140° C.         to about 160° C.; and     -   are stable across a wide pH range and thus adaptable in a number         of applications.

It is particularly interesting to note that NHNU and salts thereof exhibit long-term stability upon storage, in both light and dark conditions, in contrast to both N-hydroxyurea (NHU) and N-nitrourea (NNU), both of which readily decompose to give gaseous products. Storage stability is of course of utmost importance in any commercial application, and both NHU and NNU readily decompose to give gaseous products.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a molecule of N-hydroxyl-N′-nitrourea, synthesized and characterized as described in Example 2, as determined by x-ray crystallography.

FIG. 2 shows the crystal structure of N-hydroxyl-N′-nitrourea, synthesized and characterized as described in Example 2.

FIG. 3 illustrates a molecule of the ammonium salt of N-hydroxyl-N′-nitrourea, synthesized and characterized as described in Example 4, as determined by x-ray crystallography.

FIG. 4 illustrates the unit cell contents of N-hydroxyl-N′-nitrourea, ammonium salt, syntheszied and characterized as described in Example 4.

FIG. 5 illustrates a molecule of N-hydroxyl-N′-nitroguanidine, synthesized and characterized as described in Example 5, as determined by x-ray crystallography.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Unless otherwise indicated, the invention is not limited to specific molecular structures, analogs, composition components (e.g., igniter materials, gas-generating fuels, binders, secondary oxidizers), synthetic methods, methods of manufacture, or the like, as such may vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

As used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a compound” includes a single compound as well as a combination or mixture of two or more compounds, reference to “a substituent” includes a single substituent as well as two or more substituents that may be the same or different, and the like.

In this specification and in the claims that follow, reference will be made to a number of terms, which shall be defined to have the following meanings:

As used herein, the phrase “having the formula” or “having the structure” is not intended to be limiting and is used in the same way that the term “comprising” is commonly used.

The term “alkyl” as used herein refers to a branched or unbranched saturated hydrocarbon group typically, although not necessarily, containing 1 to about 24 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, octyl, decyl, and the like, as well as cycloalkyl groups such as cyclopentyl, cyclohexyl and the like. Generally, although again not necessarily, alkyl groups herein contain 1 to about 18 carbon atoms, preferably 1 to about 12 carbon atoms. The term “lower alkyl” intends an alkyl group of 1 to 6 carbon atoms. Preferred substituents identified as “C₁-C₆ alkyl” or “lower alkyl” contain 1 to 3 carbon atoms, and particularly preferred such substituents contain 1 or 2 carbon atoms (i.e., methyl and ethyl). “Substituted alkyl” refers to alkyl substituted with one or more substituent groups, and the terms “heteroatom-containing alkyl” and “heteroalkyl” refer to alkyl in which at least one carbon atom is replaced with a heteroatom, as described in further detail infra. If not otherwise indicated, the terms “alkyl” and “lower alkyl” include linear, branched, cyclic, unsubstituted, substituted, and/or heteroatom-containing alkyl or lower alkyl, respectively.

The term “alkenyl” as used herein refers to a linear, branched, or cyclic hydrocarbon group of 2 to about 24 carbon atoms containing at least one double bond, such as ethenyl, n-propenyl, isopropenyl, n-butenyl, isobutenyl, octenyl, decenyl, tetradecenyl, hexadecenyl, eicosenyl, tetracosenyl, and the like. Generally, although again not necessarily, alkenyl groups herein contain 2 to about 18 carbon atoms, preferably 2 to 12 carbon atoms. The term “lower alkenyl” intends an alkenyl group of 2 to 6 carbon atoms, and the specific term “cycloalkenyl”intends a cyclic alkenyl group, preferably having 5 to 8 carbon atoms. The term “substituted alkenyl” refers to alkenyl substituted with one or more substituent groups, and the terms “heteroatom-containing alkenyl” and “heteroalkenyl” refer to alkenyl in which at least one carbon atom is replaced with a heteroatom. If not otherwise indicated, the terms “alkenyl” and “lower alkenyl” include linear, branched, cyclic, unsubstituted, substituted, and/or heteroatom-containing alkenyl and lower alkenyl, respectively.

The term “alkynyl” as used herein refers to a linear or branched hydrocarbon group of 2 to 24 carbon atoms containing at least one triple bond, such as ethynyl, n-propynyl, and the like. Generally, although again not necessarily, alkynyl groups herein contain 2 to about 18 carbon atoms, preferably 2 to 12 carbon atoms. The term “lower alkynyl” intends an alkynyl group of 2 to 6 carbon atoms. The term “substituted alkynyl” refers to alkynyl substituted with one or more substituent groups, and the terms “heteroatom-containing alkynyl” and “heteroalkynyl” refer to alkynyl in which at least one carbon atom is replaced with a heteroatom. If not otherwise indicated, the terms “alkynyl” and “lower alkynyl” include linear, branched, unsubstituted, substituted, and/or heteroatom-containing alkynyl and lower alkynyl, respectively.

The term “aryl” as used herein, and unless otherwise specified, refers to an aromatic substituent containing a single aromatic ring or multiple aromatic rings that are fused together, directly linked, or indirectly linked (such that the different aromatic rings are bound to a common group such as a methylene or ethylene moiety). Preferred aryl groups contain 5 to 20 carbon atoms, and particularly preferred aryl groups contain 5 to 14 carbon atoms. Exemplary aryl groups contain one aromatic ring or two fused or linked aromatic rings, e.g., phenyl, naphthyl, biphenyl, diphenylether, diphenylamine, benzophenone, and the like. “Substituted aryl” refers to an aryl moiety substituted with one or more substituent groups, and the terms “heteroatom-containing aryl” and “heteroaryl” refer to aryl substituent in which at least one carbon atom is replaced with a heteroatom, as will be described in further detail infra. If not otherwise indicated, the term “aryl” includes unsubstituted, substituted, and/or heteroatom-containing aromatic substituents.

The term “alkaryl” refers to an aryl group with an alkyl substituent, and the term “aralkyl” refers to an alkyl group with an aryl substituent, wherein “aryl” and “alkyl” are as defined above. Preferred aralkyl groups contain 6 to 24 carbon atoms, and particularly preferred aralkyl groups contain 6 to 16 carbon atoms. Examples of aralkyl groups include, without limitation, benzyl, 2-phenyl-ethyl, 3-phenyl-propyl, 4-phenyl-butyl, 5-phenyl-pentyl, 4-phenylcyclohexyl, 4-benzylcyclohexyl, 4-phenylcyclohexylmethyl, 4-benzylcyclohexylmethyl, and the like. Alkaryl groups include, for example, p-methylphenyl, 2,4-dimethylphenyl, p-cyclohexylphenyl, 2,7-dimethylnaphthyl, 7-cyclooctylnaphthyl, 3-ethyl-cyclopenta-1,4-diene, and the like.

The terms “halo” and “halogen” are used in the conventional sense to refer to a chloro, bromo, fluoro, or iodo substituent.

The term “heteroatom-containing” as in a “heteroatom-containing alkyl group” (also termed a “heteroalkyl” group) or a “heteroatom-containing aryl group” (also termed a “heteroaryl” group) refers to a molecule, linkage, or substituent in which one or more carbon atoms are replaced with an atom other than carbon, e.g., nitrogen, oxygen, sulfur, phosphorus, or silicon, typically nitrogen, oxygen, or sulfur. Similarly, the term “heteroalkyl” refers to an alkyl substituent that is heteroatom-containing, the term “heterocyclic” refers to a cyclic substituent that is heteroatom-containing, the terms “heteroaryl” and “heteroaromatic respectively refer to “aryl” and “aromatic” substituents that are heteroatom-containing, and the like. Examples of heteroalkyl groups include alkoxyaryl, alkylsulfanyl-substituted alkyl, N-alkylated amino alkyl, and the like. Examples of heteroaryl substituents include pyrrolyl, pyrrolidinyl, pyridinyl, quinolinyl, indolyl, pyrimidinyl, imidazolyl, 1,2,4-triazolyl, tetrazolyl, etc., and examples of heteroatom-containing alicyclic groups are pyrrolidino, morpholino, piperazino, piperidino, etc.

By “substituted,” as in “substituted alkyl,” “substituted aryl,” and the like, as alluded to in some of the aforementioned definitions, is meant that in the alkyl, aryl, or other moiety, at least one hydrogen atom bound to a carbon (or other) atom is replaced with one or more non-hydrogen substituents. Examples of such substituents include, without limitation: functional groups such as halo, hydroxyl, sulfhydryl, C₁-C₂₄ alkoxy, C₂-C₂₄ alkenyloxy, C₂-C₂₄ alkynyloxy, C₅-C₂₀ aryloxy, acyl (including C₂-C₂₄ alkylcarbonyl (—CO-alkyl) and C₆-C₂₀ arylcarbonyl (—CO-aryl)), acyloxy (—O-acyl), C₂-C₂₄ alkoxycarbonyl (—(CO)-O-alkyl), C₆-C₂₀ aryloxycarbonyl (—(CO)-O-aryl), halocarbonyl (—CO)-X where X is halo), C₂-C₂₄ alkylcarbonato (—O—(CO)—O-alkyl), C₆-C₂₀ arylcarbonato (—O—(CO)—O-aryl), carboxy (—COOH), carboxylato (—COO⁻), carbamoyl (—(CO)—NH₂), mono-(C₁-C₂₄ alkyl)-substituted carbamoyl (—(CO)—NH(C₁-C₂₄ alkyl)), di-(C₁-C₂₄ alkyl)-substituted carbamoyl (—(CO)—N(C₁-C₂₄ alkyl)₂), mono-substituted arylcarbamoyl (—(CO)—NH-aryl), thiocarbamoyl (—(CS)—NH₂), carbamido (—NH—(CO)—NH₂), cyano(—C≡N), isocyano (—N⁺≡C⁻), cyanato (—O—C≡N), isocyanato (—O—N⁺≡C⁻), isothiocyanato (—S—C≡N), azido (—N═N⁺═N⁻), formyl (—(CO)—H), thioformyl (—(CS)—H), amino (—NH₂), mono- and di-(C₁-C₂₄ alkyl)-substituted amino, mono- and di-(C₅-C₂₀ aryl)-substituted amino, C₂-C₂₄ alkylamido (—NH—(CO)-alkyl), C₆-C₂₀ arylamido (—NH—(CO)-aryl), imino (—CR═NH where R=hydrogen, C₁-C₂₄ alkyl, C₅-C₂₀ aryl, C₆-C₂₄ alkaryl, C₆-C₂₄ aralkyl, etc.), alkylimino (—CR═N(alkyl), where R=hydrogen, alkyl, aryl, alkaryl, etc.), arylimino (—CR═N(aryl), where R=hydrogen, alkyl, aryl, alkaryl, etc.), nitro (—NO₂), nitroso (—NO), sulfo (—SO₂—OH), sulfonato (—SO₂—O⁻), C₁-C₂₄ alkylsulfanyl (—S-alkyl; also termed “alkylthio”), arylsulfanyl (—S-aryl; also termed “arylthio”), C₁-C₂₄ alkylsulfinyl (—(SO)-alkyl), C₅-C₂₀ arylsulfinyl (—(SO)-aryl), C₁-C₂₄ alkylsulfonyl (—SO₂-alkyl), C₅-C₂₀ arylsulfonyl (—SO₂-aryl), phosphono (—P(O)(OH)₂), phosphonato (—P(O)(O⁻)₂), phosphinato (—P(O)(O⁻)), phospho (—PO₂), and phosphino (—PH₂); and the hydrocarbyl moieties C₁-C₂₄ alkyl (preferably C₁-C₁₈ alkyl, more preferably C₁-C₁₂ alkyl, most preferably C₁-C₆ alkyl), C₂-C₂₄ alkenyl (preferably C₂-C₁₈ alkenyl, more preferably C₂-C₁₂ alkenyl, most preferably C₂-C₆ alkenyl), C₂-C₂₄ alkynyl (preferably C₂-C₁₈ alkynyl, more preferably C₂-C₁₂ alkynyl, most preferably C₂-C₆ alkynyl), C₅-C₂₀ aryl (preferably C₅-C₁₄ aryl), C₆-C₂₄ alkaryl (preferably C₆-C₁₈ alkaryl), and C₆-C₂₄ aralkyl (preferably C₆-C₁₈ aralkyl).

In addition, the aforementioned functional groups may, if a particular group permits, be further substituted with one or more additional functional groups or with one or more hydrocarbyl moieties such as those specifically enumerated above. Analogously, the above-mentioned hydrocarbyl moieties may be further substituted with one or more functional groups or additional hydrocarbyl moieties such as those specifically enumerated.

In one embodiment, then, the compounds of the invention are salts having the structure of formula (I)

wherein the various substituents are as follows:

R¹ is H, C₁-C₂₄ alkyl (preferably C₁-C₁₂ alkyl, more preferably C₁-C₆ alkyl), C₂-C₂₄ alkenyl (preferably C₂-C₁₂ alkenyl, more preferably C₂-C₆ alkenyl), C₂-C₂₄ alkynyl (preferably C₂-C₁₂ alkynyl, more preferably C₂-C₆ alkynyl), C₅-C₂₀ aryl (preferably C₅-C₁₄ aryl), C₆-C₂₄ alkaryl (preferably C₆-C₁₆ alkaryl), or C₆-C₂₄ aralkyl (preferably C₆-C₁₆ aralkyl), wherein any of the aforementioned hydrocarbyl groups are optionally substituted and/or heteroatom-containing. Most preferably, R is H.

R² is H, C₁-C₂₄ alkyl (preferably C₁-C₁₂ alkyl, more preferably C₁-C₆ alkyl), or nitro. Most preferably, R² is nitro.

X is O or NR³ in which R³ is H, C₁-C₂₄ alkyl (preferably C₁-C₁₂ alkyl, more preferably C₁-C₆ alkyl) or nitro. Most preferably X is O or NR³ wherein R³ is nitro.

Z^(m+) is a cation selected from a monovalent cation, a divalent cation, and a trivalent cation, wherein m and n are 1, 2, or 3. In this embodiment, a particularly preferred compound is in the form of an ammonium salt, wherein m is 1, n is 1, and Z is NH₄.

In addition, the compound of formula (I) contains at least one nitro group.

In a preferred compound of formula (I), R¹ is H, X is O, and R² is nitro, such that the compound is a salt of the N-hydroxyl-N′-nitrourea monoanion and a cation Z^(m+), having the structure of formula (Ia)

In a particularly preferred embodiment, m and n are 1, and Z is ammonium, such that the compound is the ammonium salt of the N-hydroxyl-N′-nitrourea monoanion.

In another embodiment, the invention provides a salt having the structure of formula (II)

wherein R¹, R² and X are as defined for formula (I), and M is an alkali metal such as sodium or potassium. Most preferably, M is sodium.

In a preferred compound of formula (II), R¹ is H, X is O, and R² is nitro, such that the compound is a salt of the N-hydroxyl-N′-nitrourea dianion and a cation M⁺, having the structure of formula (Ia)

In a particularly preferred embodiment, M is sodium, and the compound is the disodium salt of the N-hydroxyl-N′-nitrourea dianion.

In another embodiment, the invention is directed to N-hydroxyl-N′-nitrourea, as a new composition of matter, in electronically neutral form. The compound has the structure of formula (III)

In another embodiment, the invention is directed to N-hydroxyl-N-nitroguanidine, in electronically neutral form, also as a new composition of matter. The compound has the structure of formula (IV)

Energetic compositions of the invention contain a compound of the invention as provided herein, a binder, and, optionally, one or more additional energetic materials. The binder will typically be an organic polymeric material, and may be either inert or energetic. Generally, the invention polymeric binder represents in the range of approximately 5 wt. % to 50 wt. % of the composition, preferably 10 wt. % to 30 wt. % of the composition, while the energetic material(s) represent approximately 50 wt. to 95 wt. % of the composition, preferably 70 wt. % to 90 wt. % of the composition, with one or more compounds of the invention representing 50 wt. % to 100 wt. %, preferably 75 wt. % to 100 wt. %, of the total energetic materials in the composition.

Examples of inert binders include, without limitation: poly(alkylene glycols) such as polyethylene glycol (PEG) and poly(ethylene-co-propylene) glycol; polybutadienes such as hydroxy-terminated polybutadiene (HTPB) and butadiene acrylonitrile-acrylic acid terpolymer (PBAN); polyesters; polyacrylates; polymethacrylates; polycarbonates; and polyurethanes. Examples of energetic polymeric binder materials for use in propellant applications include, but are not limited to: polyoxetanes such as nitratomethyl-methyloxetane (poly-NMMO), poly(bisazidomethyl-oxetane) (poly-BAMO), poly(azidomethyl-methyloxetane)(poly-AMMO), poly(nitraminomethyl-methyloxetane) (poly-NAMMO), poly (BAMO-co-NMMO), and poly(BAMO-co-AMMO); polyglycidyl azide (GAP); and polyglycidyl nitrate (PGN). Other suitable binder materials will be known to those skilled in the art and are described in the pertinent literature.

It will also be appreciated that the energetic compositions may also include one or more plasticizers for the polymeric binder. The plasticizer selected will depend on the particular binder polymer(s) and on the desired or necessary properties of the energetic composition. In general, however, plasticizers used in conjunction with the above-mentioned binders include dioctyladipate, isodecylperlargonate, dioctylphthalate, dioctylmaleate, and dibutylphthalate, as well as the energetic plasticizers bis(2,2-dinitropropyl)acetal/bis(2,2-dinitropropyl)formal (BDNPF/BDNPA), trimethylolethanetrinitrate (TMETN), butanetrioltrinitrate (BTTN), nitroglycerine (NG), diethyleneglycoldinitrate (DEGN), and triethyleneglycoldinitrate (TEGDN). Some binders will also require curing agents, such as isocyanates or the like (e.g., hexamethylene diisocyanate).

Additional energetic materials that may be incorporated into the present energetic compositions include conventional explosive materials such as TNT (2,4,6-trinitrotoluene), RDX (cyclo-1,3,5-tri-methylene-2,4,6-trinitramine; also referred to as cyclonite), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.0.^(5,9).0^(3,11)]-dodecane), TEX (4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo[5.5.0.0.^(5,9).0^(3,11)]-dodecane), NTO (3-nitro-1,2,4-triazol-5-one), TATB (1,3,5-triamino-2,4,6-trinitrobenzene), TNAZ (1,3,3-trinitroazetidine), NQ (nitroguanidine), DADNE (1,1-diamino-2,2-dinitroethane), and picric acid.

Depending on intended use, the energetic compositions of the invention will contain additional components, such as a fuel, an igniter compound (or “initiator”), additional oxidizers, and the like.

For example, castable and extrudable explosive formulations will contain a reactive metal as a fuel, e.g., e.g., aluminum, beryllium, boron, magnesium, titanium, zirconium, or mixtures or alloys thereof, and one or more oxidizers in addition to the inventive compound.

Preferred igniter compounds for use herein are thermally stable, typically up to a temperature of at least about 150° C.; the compounds should also have a relatively high heat of formation, and be safe, economical, and straightforward to synthesize in relatively high yield. Conventional igniters such as lead azide and lead styphnate may be used, although preferred igniters are the N,N′-azobis-nitroazoles described in commonly assigned U.S. Pat. No. 5,889,161 to Bottaro et al. for “N,N-Azobis-Nitroazoles and Analogs Thereof as Igniter Compounds for Use in Energetic Compositions,” assigned to SRI International (Menlo Park, Calif.).

Examples of additional oxidizers that may be incorporated into the energetic compositions include, but are not limited to, ammonium nitrate (AN), phase-stabilized ammonium nitrate (PSAN), ammonium dinitramide (ADN), potassium nitrate (KN), potassium dinitramide (KDN), sodium peroxide (Na₂O₂), ammonium perchlorate (AP), hydroxylammonium nitrate (HAN), and KDN-AN, a cocrystallized form of potassium dinitramide and ammonium nitrate.

A particular application of such energetic materials is in the manufacture of propellants, e.g., rocket propellants, including both solid and solution propellants. That is, such compositions will contain, in addition to a secondary explosive comprising a compound of the invention and an igniter material as described above, an inert or energetic binder, and a metallic fuel. Other components for incorporation into propellants include burn rate modifiers, ballistic additives, and the like.

A compound of the invention may also be used as the liquid oxidizer component of monopropellant, bipropellant and tripropellant compositions, without need for additional oxidizing agents. In addition, the compounds are useful in pyrotechnic applications, in firearms, and the like.

Another application of the present compounds is in gas-generating compositions for inflating airbags in automobiles, planes, and the like. These gas-generating compositions contain a compound of the invention, an igniter material as described above, and, optionally, an additional oxidizer such as AN, PSAN, ADN, KN, KDN, Na₂O₂, AP, HAN, or KDN-AN. Gas-generating compositions may also, if desired, contain a gas-generating fuel and a binder, either an inert or an energetic binder as described previously. Suitable gas-generating fuels are nitrogenous compounds, including, by way of example, triaminoguanidine nitrate (TAGN), diaminoguanidine nitrate (DAGN), monoaminoguanidine nitrate (MAGN), 3-nitro-1,2,4-triazole-5-one (NTO), salts of NTO, urazole, triazoles, tetrazoles, guanidine nitrate, oxamide, oxalyldihydrazide, melamine, various pyrimidines, semicarbazide (H₂N—(CO)—NHNH₂), azodicarbonamide (H₂N—(CO)—N═N—(CO)—NH₂), and mixtures thereof.

The compounds of the invention may be readily synthesized in a variety of ways using techniques that are straightforward and readily scaled up. Compounds of formulae (I), (II) and (III) may conveniently be synthesized from a lower alkyl carbamate such as methyl carbamate, which is available from Aldrich (Milwaukee, Wis.) and other commercial sources. The carbamate can be treated with equivalent molar amounts of acetyl nitrate and acetic acid to form a lower alkyl nitrocarbamate, e.g., methyl nitrocarbamate (for example by using the procedure described in Example 1 below), or ethyl nitrocarbamate (also known as nitrourethane). Ethyl nitrocarbamate can also be obtained commercially, e.g., from Aldrich. Compounds of formula (I) may then be synthesized by treating the lower alkyl nitrocarbamate with a hydroxide-releasing base (e.g., sodium hydroxide or potassium hydroxide) in the presence of aqueous hydroxylamine until a salt of di-anionic N-hydroxyl-N′-nitrourea or an analog thereof precipitates (e.g., the sodium or potassium salt). The salt may, if desired, be converted to electronically neutral form (e.g., to provide a compound of formula (III)), by processing the precipitate (which may or may not be isolated) with a solution of a strong inorganic acid, e.g., a sulfuric acid solution. The compound thus obtained—i.e., NHNU or an analog thereof—may then be obtained by filtration, and the isolated filtrate then dissolved in a solvent, e.g. acetonitrile, and crystallized.

The ammonium salts of NHNU and NHNU analogs may be formed by direct reaction of NHNU with ammonia, for example, by adding ammonia to a methanol solution of NHNU in a sufficient amount to neutralize the solution. The resulting precipitated ammonium salt of the NHNU monoanion, a compound of formula (II), may then be filtered and dried.

The compound of formula (IV), N-hydroxyl-N′-nitroguanidine, may be synthesized by reacting thiourea with dimethyl sulfate to provide the sulfate salt of 2-methyl-isothiourea, followed by nitration with a mixture of nitric acid and sulfuric acid. The resulting intermediate, N-nitro-2-methyl-isothiourea, is then converted to N-hydroxyl-N′-nitroguanidine with hydroxylamine and precipitated from ethanol. The reaction is illustrated in Scheme 1:

Alternatively, the compound of formula (IV), N-hydroxyl-N′-nitroguanidine, may be directly synthesized from N-nitroguanidine as indicated in Scheme 2:

Synthetic details not explicitly disclosed herein are within the knowledge of or may be deduced by one skilled in the art of synthetic organic chemistry, or may be found in, e.g., Kirk-Othmer's Encyclopedia of Chemical Technology, House's Modern Synthetic Reactions, C. S. Marvel and G. S. Hiers' text, ORGANIC SYNTHESIS, Collective Volume 1, or in T. L. Gilchrist, Heterocyclic Chemistry, 2nd Ed. (New York: John Wiley & Sons, 1992), or the like. Synthesis of representative compounds is exemplified below

Manufacture of gas-generating compositions, propellants, and other energetic compositions may be carried out using conventional means, as will be appreciated by those skilled in the art. A suitable method for preparing anhydrous gas-generating compositions is disclosed, for example, in U.S. Pat. No. 5,473,647 to Blau et al. and in international patent publication WO 95/00462 (Poole et al.).

Of course, other methods for manufacture may be used as well. Such methods are described in the pertinent literature or will be known to those familiar with the preparation of energetic compositions.

It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, the description above as well as the examples that follow are intended to illustrate and not limit the scope of the invention. Other aspects, advantages, and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.

All patents, patent applications, journal articles, and other reference cited herein are incorporated by reference in their entireties.

EXPERIMENTAL

The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to prepare and use the compounds disclosed and claimed herein. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.) but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in degrees Celsius (C), and pressure is at or near atmospheric.

Example 1 Synthesis of N-Hydroxyl-N′-Nitrourea, Dipotassium Salt

(a) Preparation of methyl nitrocarbamate: Methyl carbamate (0.5 mole) (obtained from Aldrich) was dissolved in 500 mL of CHCl₃ and cooled to 0° C. This mixture was treated with a solution of 0.5 mole acetyl nitrate, 0.5 mole acetic acid, and 200 mL chloroform over 1 hour. The mixture was allowed to warm overnight and was concentrated in vacuo, drying under high vacuum.

(b) Methyl nitrocarbamate (100 mmol) was dissolved in 80 mL of methanol and cooled to 0° C. A solution of 100 mmol potassium hydroxide in 20 mL of methanol was added with stirring (forming potassium methyl nitrocarbamate). Next, a solution of 100 mmol of potassium hydroxide, 120 mmol of hydroxylamine, and 50 mL of methanol was added, with stirring. A dense, yellow precipitate appeared, which was the di-anion of N-hydroxyl-N′-nitrourea (NHNU), as its potassium salt.

Example 2 Synthesis of N-Hydroxyl-N′-Nitrourea

The reaction mixture containing the NHNU dipotassium salt obtained in part (b) of Example 1 was stirred for 3 hours at 0° C., and then neutralized with 110 mmol of H₂SO₄, mixed with 500 mL 2-propanol, filtered, and the filtrate concentrated to dryness. The crude residue was crystallized from 500 mL hot acetonitrile to give 8.0 g of NHNU in the form of orthorhombic prisms. The crystals were found to decompose at about 145° C.

Differential scanning calorimetry (DSC) was performed on the obtained NHNU using a TA Instruments DSC, scanned from 30° C. to 250° C. at 10° C. per minute on a sample weighing 0.200 mg. A single, very strong exothermic peak was produced, at about 156.5° C. The area under this peak indicated an energy content of about 2445 J/g.

The crystal structure of the obtained NHNU was determined by x-ray crystallography at 293(2) K. A single crystal of NHNU, about 0.40×0.18×0.23 mm³, was used for the analysis. The crystal was determined to be orthorhombic, space group P2₁2₁2₁ (no. 19), unit cell dimensions a=4.7116(4) Å, b=6.9585(6) Å, c=12.8495(11) Å (where the numbers in parentheses are the estimated standard deviations for the least significant digits). There are four formula units of CH₃N₃O₄ in the unit cell, leading to a calculated density of about 1.91 g/cm³. The determined structure of the NHNU molecule is shown in FIG. 1, and the crystal structure is shown in FIG. 2. It is noted that different crystal structures may be obtained under different conditions of crystal growth.

Example 3 Evaluation of N-Hydroxyl-N′-Nitrourea

Various properties of NHNU (in electronically neutral form) were evaluated using standard equipment and procedures, and the results are set forth in Table 1:

TABLE 1 PROPERTY EVALUATED RESULT Density 1.909 g/cc (x-ray diffraction) ABL Impact 11 cm ABL Friction 50 lb @ 8 ft/sec TC Impact 19.5 cm (H₅₀) (1 kg drop weight test) TC ESD Unconfined >8 Joules SBAT Onset 193° F. exotherm Iso SBAT @ 225° F., immediate exotherm >30° F.

Example 4 Synthesis of N-Hydroxyl-N′-Nitrourea, Ammonium Salt

The ammonium salt of NHNU was synthesized as follows: N-hydroxyl-N′-nitrourea (16 mmol) was dissolved in 25 mL of methanol at 20° C. One equivalent of ammonia (0.5 M in 1,4-dioxane) was added, and a precipitate appeared immediately. After 20 minutes of stirring the solid was isolated by filtration, washed with 10 mL methanol, and air dried to give 2.0 g of ammonium N-hydroxyl-N′-nitrourea. The salt was slightly soluble in hot methanol (about 0.3 g in 100 mL of methanol).

Differential scanning calorimetry (DSC) was performed on the obtained ammonium-NHNU using a TA Instruments DSC, scanned from 30° C. to 220° C. at 10° C. per minute on a sample weighing 0.200 mg. A single, very strong exothermic peak was produced, at about 157° C. The area under this peak indicated an energy content of about 1541 J/g.

The crystal structure of the obtained ammonium salt of NHNU was determined by x-ray crystallography at 293(2) K. A single crystal of this salt, about 0.24×0.14×0.04 mm³, was used for the analysis. The crystal was determined to be triclinic, space group P-1 (no. 2), unit cell dimensions a=5.3765(4) Å, b=6.7723(5) Å, c=7.9155(5) Å, α=109.873(4)°, β=100.217(4)°, γ=102.520(4)°. There are two formula units of CH₆N₄O₄ in the unit cell, leading to a calculated density of about 1.80 g/cm³. The determined structure of the ammonia salt of the NHNU molecule is shown in FIG. 3, and the unit cell contents are shown in FIG. 4. It is noted that different crystal structures may be obtained under different conditions of crystal growth.

Example 5 Synthesis of N-Hydroxyl-N′-Nitroguanidine

The compound of formula (IV), N-hydroxyl-N′-nitroguanidine, was synthesized by reacting thiourea with dimethyl sulfate to provide the sulfate salt of 2-methyl-isothiourea, followed by nitration with a mixture of nitric acid and sulfuric acid (as described in the Journal of the American Chemical Society 76 :1877 (1954). The resulting intermediate, N-nitro-2-methyl-isothiourea (37 mmol) was combined with 37 mmol of hydroxylamine in 75 mL of ethanol. After stirring for 4 hours the precipitate was isolated by filtration, washed with 20 mL of ethanol and air-dried to give N-hydroxyl-N′-nitroguanidine in 85% yield.

Differential scanning calorimetry (DSC) was performed on the obtained compound using a TA Instruments DSC, scanned from 30° C. to 220° C. at 10° C. per minute on a sample weighing 0.210 mg. A single, very strong exothermic peak was produced, at about 114° C. The area under this peak indicated an energy content of about 1873 J/g.

The crystal structure of the compound was determined by x-ray crystallography at 293(2) K. A single crystal of this salt, about 0.08×0.08×0.24 mm³, was used for the analysis. The crystal was determined to be monoclinic, space group C2/c, unit cell dimensions a=17.9449(2) Å, b=4.22690(10) Å, c=11.89120(10) Å, α=90°, β=99.9270(10)°, γ=90°. Calculated density: 1.795 g/cm³. The determined structure of the compound is shown in FIG. 5.

Example 6 Preparation and Testing of Propellant Compositions

NHNU was mixed with several combinations of propellant components, as indicated in Table 1. Each of these mixtures was placed in a differential scanning calorimeter (DSC) and held at 80° C. for 24 h. The times and magnitudes of any resulting exotherms or endotherms were recorded and shown in Table 2. Unless otherwise indicated, the typical first exotherm was a somewhat broad but large peak, taking about three hours to fully develop, which was followed by a sharp spike about 15 minutes in duration. The largest exotherms, representing the largest energy releases, generally occurred when NHNU was mixed with ammonium nitrate (AN).

NHNU was mixed with several combinations of potential propellant components, as indicated in Table 1. Each of these mixtures was placed in a differential scanning calorimeter (DSC) and held at 80° C. for 24 h. The times and magnitudes of any resulting exotherms or endotherms were recorded, as shown in Table 2. Unless otherwise indicated, the typical first exotherm was a somewhat broad but large peak, taking about three hours to fully develop, which was followed by a sharp spike about 15 minutes in duration. The largest exotherms, representing the largest energy releases, generally occurred when NHNU was mixed with ammonium nitrate (AN).

TABLE 2 Thermal behavior of NHNU in combination with other materials at 80° C.: MIXTURE TIME TO EXOTHERM(S) NHNU, PGN, BTTN 16 h, 17 h NHNU, BTTN, AN 7 h, 9 h; endotherm at 5 h NHNU, PGN, AN 4 h, 5.5 h, 7.25 h; endotherm at 4.25 h NHNU, PGN, ADN Jagged base line; no definite exotherm NHNU, PGN, CL-20 15 h, 16 h NHNU, PGN, N-100 No exotherm; endotherm at 3.25 h NHNU, BTTN, ADN Jagged base line; slow exotherm at 14 h NHNU, BTTN, CL-20 Slight at 2 h, 6 h NHNU, BTTN, N-100 Slow, slight at 7 h, 8.5 h NHNU, AN, ADN 2 h, 5 h NHNU, AN, CL-20 5 h, 7 h; endotherm at 3.5 h NHNU, AN, N-100 No exotherm; endotherms at 2.5 h, 9 h NHNU, ADN, CL-20 Slow, mild at 14 h; mild at 16 h NHNU, ADN, N-100 No exotherm; endotherms at 2 h, 9 h NHNU, CL-20, N-100 No exotherm; endotherm at 8 h

In Table 2, the abbreviations used are as follows: ADN=ammonium dinitramide; AN=ammonium nitrate; BTTN=butanetrioltrinitrate; CL-20=2,4,6,8,10,12-hexanitrohexaaza-isowurtzitane; N-100=aliphatic polyisocyanate resin based on hexamethylene diisocyanate, available from Bayer Corporation, Pittsburgh, Pa. as Desmodur® N100; NHNU=N-hydroxyl-N′-nitrourea; PGN=polyglycidylnitrate. 

1. A compound in the form of a salt having the structure of formula (I)

wherein: R¹ is selected from the group consisting of H, C₁-C₂₄ alkyl, C₂-C₂₄ alkenyl, C₂-C₂₄ alkynyl, C₅-C₂₀ aryl, C₆-C₂₄ alkaryl, and C₆-C₂₄ aralkyl; R² is H, C₁-C₂₄ alkyl or nitro; X is O or NR³ in which R³ is H, C₁-C₂₄ alkyl or nitro; Z^(m+) is a monovalent cation, a divalent cation, or a trivalent cation; and m and n are 1, 2 or 3, with the proviso that the compound contains at least one nitro group.
 2. The compound of claim 1, wherein R¹ is H, R² is nitro, X is O, such that the compound is a salt of N-hydroxyl-N′-nitrourea monoanion.
 3. The compound of claim 2, wherein m and n are 1, and Z is ammonium ion, such that the compound is N-hydroxyl-N′-nitrourea, ammonium salt.
 4. A compound in the form of a salt having the structure of formula (II)

wherein: R¹ is selected from the group consisting of H, C₁-C₂₄ alkyl, C₂-C₂₄ alkenyl, C₂-C₂₄ alkynyl, C₅-C₂₀ aryl, C₆-C₂₄ alkaryl, and C₆-C₂₄ aralkyl; R² is H, C₁-C₂₄ alkyl or nitro; X is O or NR³ in which R³ is H, C₁-C₂₄ alkyl or nitro; and M is an alkali metal, with the proviso that the compound contains at least one nitro group.
 5. The compound of claim 4, wherein R¹ is H, R² is nitro, X is O, and M is potassium, such that the compound is N-hydroxyl-N′-nitrourea, dipotassium salt.
 6. N-hydroxyl-N′-nitrourea.
 7. N-hydroxyl-N′-nitroguanidine.
 8. An energetic composition comprising the compound of claim 2 and a polymeric binder.
 9. An energetic composition comprising the compound of claim 3 and a polymeric binder.
 10. An energetic composition comprising the compound of claim 5 and a polymeric binder.
 11. A propellant composition comprising the compound of claim 2, a polymeric binder, a metallic fuel, and an igniter compound.
 12. A propellant composition comprising the compound of claim 3, a polymeric binder, a metallic fuel, and an igniter compound.
 13. A propellant composition comprising the compound of claim 5, a polymeric binder, a metallic fuel, and an igniter compound.
 14. A gas-generating composition comprising the compound claim 2, a polymeric binder, a nitrogenous fuel, and an igniter compound.
 15. A gas-generating composition comprising the compound claim 3, a polymeric binder, a nitrogenous fuel, and an igniter compound.
 16. A gas-generating composition comprising the compound claim 5, a polymeric binder, a nitrogenous fuel, and an igniter compound.
 17. A method for synthesizing a salt of N-hydroxyl-N′-nitrourea, comprising treating a basic salt of a lower alkyl nitrocarbamate and a monovalent cation with a hydroxide-releasing base in the presence of aqueous hydroxylamine until a salt of the monovalent cation and di-anionic N-hydroxyl-N′-nitrourea precipitates.
 18. A method for synthesizing N-hydroxyl-N′-nitrourea, comprising carrying out the method of claim 17 and thereafter processing the precipitate with a sulfuric acid solution to obtain N-hydroxyl-N′-nitrourea in electronically neutral form.
 19. A method for synthesizing the ammonium salt of N-hydroxyl-N′-nitrourea, comprising: carrying out the method of claim 18; and re-dissolving the N-hydroxyl-N′-nitrourea in a solvent containing a stoichiometric amount of ammonia. 